1. Field of the Invention
The present invention relates to activated carbon honeycomb catalyst beds for removing mercury and/or other toxic metals from fluid process streams and systems comprised thereof.
2. Technical Background
Mercury is both a global pollutant and a contaminant that can be transformed to a potentially toxic species (methylmercury) under natural conditions. Mercury emitted to the atmosphere can travel thousands of miles before being deposited to the earth. Studies show that mercury from the atmosphere can also be deposited in areas near an emission source. According to a National Academy of Sciences study published in July, 2001, there are about 60,000 children, who are born in the USA, potentially affected by mercury toxicity every year. It has been reported that human inhalation of elemental mercury has acute effects on kidneys and the central nervous system (CNS), such as mild transient proteinuria, acute renal failure, tremors, irritability, insomnia, memory loss, neuromuscular changes, headaches, slowed sensory, motor nerve function, and reduction in cognitive function. Acute inhalation of elemental mercury can also affect gastrointestinal and respiratory systems, causing chest pains, dyspnea, cough, pulmonary function impairment, and interstitial pneumonitis. Study also indicates that chronic exposure of elemental mercury can cause the adverse effects on kidneys and CNS including erethism (increased excitability), irritability, excessive shyness, insomnia, severe salivation, gingivitis, tremors, and the development of proteinuria. Children exposed to elemental mercury compounds have been found to have acrodynia that is characterized by severe leg cramps, irritability, paresthesia (a sensation of prickling on the skin), and painful pink fingers and peeling hands, feet, and nose. Reference Concentration (RfC) for elemental mercury exposure set by EPA is 0.0003 mg/m3, which is based on CNS effects in humans. Continuous exposure above the RfC level increases potential for adverse health effects. The main route of human exposure to methylmercury is the diet, such as eating fish. Acute exposure of methylmercury can cause CNS effects such as blindness, deafness, and impaired level of consciousness. Chronic exposure of methylmercury results in symptoms such as paresthesia (a sensation of prickling on the skin), blurred vision, malaise, speech difficulties, and constriction of the visual field. It is estimated that the minimum lethal dose of methylmercury for a 70-kg person ranges from 20 to 60 mg/kg.
Coal-fired power plants and medical waste incineration are major sources of human activity related mercury emission to the atmosphere. It is estimated that there are 48 tons of mercury emitted from coal-fired power plants in the US annually. DOE-Energy Information Administration annual energy outlook projects that coal consumption for electricity generation will increase from 976 million tons in 2002 to 1,477 million tons in 2025 as the utilization of existing and added coal-fired generation capacity increases. However, there has not been mercury emission control regulation enforced for coal-fired power plants. A major reason is that there is not an effective control technology available for a reasonable cost, especially for elemental mercury control.
The state of the art technology that has shown promise for controlling elemental mercury as well as oxidized mercury is active carbon injection (ACI). The method was disclosed early in U.S. Pat. No. 4,889,698. The ACI process includes injecting active carbon powder into the flue gas stream and using fabric fiber (FF) or electrostatic precipitator (ESP) to collect the active carbon powder that has adsorbed mercury. A pilot scale test of ACI-FF with the Norit Darco FGD carbon at a DOE/NETL test facility demonstrated that total mercury removal rate enhanced from 40% to 90% when ACI injection C:Hg ratio increased from 2,600:1 to 10,300:1. Comparison tests at the DOE/NETL facility showed that ACI-ESP could only achieve 70% mercury control at several times higher C:Hg ratio. Generally, ACI technologies require a high C:Hg ratio to achieve the desired mercury removal level (>90%), which results in a high portion cost for sorbent material. The high C:Hg ratio means that ACI does not utilize the mercury sorption capacity of carbon powder efficiently. A major problem associated with ACI technology is cost. If only one particle collection system is used, the commercial value of fly ash is sacrificed due to its mixing with contaminated activated carbon powder. Based on the cost estimation of DOE, the commercial value and disposal cost of fly ash is about 6.7 million dollars. U.S. Pat. No. 5,505,766 disclosed a method of using a system with two separate powder collectors and injecting activated carbon sorbent between the first collector for fly ash and the second collector, or a baghouse, for activated carbon powder. U.S. Pat. No. 5,158,580 described a baghouse with high collection efficiency. DOE estimation shows that the installation of additional baghouse for activated carbon powder collection costs about $28 million dollars, which is high especially for small companies.
Since water-soluble (oxidized) mercury is the main mercury species in bituminous coal flue gas with high concentrations of SO2 and HCl, bituminous coal-fired plants may be able to remove 90% mercury using a wet scrubber combined with NOx and/or SO2 control technologies. Mercury control can also be achieved as a co-benefit of particulate control. U.S. Pat. No. 6,328,939 disclosed a method of adding a chelating agent to a wet scrubbing solution because wet scrubber captured mercury can be reemitted. However, a chelating agent adds the cost due to the problems of corrosion of the metal scrubber equipment and treatment of the chelating solution. Removing oxidized mercury as a co-benefit of using a wet scrubber by injecting a calcium compound to remove SO2 was disclosed in U.S. Pat. No. 4,956,162. However, elemental mercury is the dominant species in the flue gas of sub-bituminous coal or lignite coal and a wet scrubber is not effective for removal of elemental mercury unless additional chemicals are added to the system. Injection of activated carbon into a system containing SCR and SO2 control equipment was disclosed in U.S. Pat. No. 6,610,263 and U.S. Pat. No. 6,579,507. U.S. Pat. No. 6,503,470 described a method of adding sulfide-containing liquors to the flue gas stream and U.S. Pat. No. 6,790,420 described a method of adding ammonia and optionally carbon monoxide to enhance the oxidation of mercury at 900° F. and 1300° F. However, it is undesirable to add additional materials, potentially environmentally hazardous, into the flue gas system.
An activated carbon fixed bed can reach high mercury removal level with more effective utilization of sorbent material. However, a normal powder or pellet packed bed has very high pressure drop, which significantly reduces energy efficiency. Further, these fixed beds are generally an interruptive technology because they require frequent replacement of the sorbent depending on the sorption capacity. Accordingly, reducing the pressure drop and significantly increasing the mercury sorption capacity would allow the fix bed technology to be more practical and economical to the power plant users.